With tightening environmental regulations, there is an increasing need for the measurement of particle emissions. In particular, the need for measurement is present in the development of filtering methods, in the research of various combustion processes, as well as in processes for monitoring actual emissions. One significant parameter in the measurement of particle emissions is particle density. The particle density is an important factor in a variety of properties which are significant in view of the particle being carried along, including for example the settling velocity of the particle. For this reason, the particle density is significant, for example, in the health effects of the particles, such as the accumulation of the particles in the lungs.
Problems involved in the measurement of the particle density are described, for example, in the article by W. P. Kelly and P. H. McMurry, “Measurement of Particle Density by Intertial Classification of Differential Mobility Analyzer-Generated Monodisperse Aerosol” [Aerosol Science and Technology 17: 199–212, 1992]. The same article also teaches a method of prior art for the measurement of particle density by means of a DMA device (Differential Mobility Analyzer) and an impactor. FIG. 1 shows the principle of operation of this method.
In the method disclosed in the article, a flow 13a carrying a particle distribution to be analyzed is led to an apparatus 10 consisting of a DMA device 11 and an impactor 12. The flow is first led to the DMA device 11 which, by means of an electrical field, separates the particles with a narrow electrical range of mobility from the flow to a flow 13b to be led to the impactor 12. Particles whose electrical mobility is not within this narrow range are guided with flows 13c and 13d away from the measuring device 10.
By means of the DMA device, it has thus been possible to separate the monodispersive aerosol flow 13b having a given narrow electrical mobility distribution 14b, from the polydispersive aerosol flow 13a having an electrical distribution 14a and being led to the measuring device.
This monodispersive aerosol flow is then led to the impactor 12 which, in a way known as such, classifies them on the basis of their aero-dynamic diameter, collecting particles with different aerodynamic diameters on different collection plates. By measuring the collection plates, it is possible to determine the aerodynamic size distribution 15 of the particles contained in the flow 13b input in the impactor.
When both the electrical mobility diameter and the aerodynamic size distribution of the particle distribution to be analyzed are known, it is possible to compute the density of the particle distribution in the way presented in the article.
The above-presented solution of prior art involves the problem that the density can -only be determined for a narrow electrical mobility range at a time. In other words, by means of the method, the density can be computed for the monodispersive flow 13b by means of the DMA device. To determine the density distribution of the particles in the polydispersive flow 13a, this must be implemented, according to the above-presented solution of prior art, by scanning, i.e. by first determining the density in one electrical mobility range and then changing the adjustments of the DMA device in such a way that the measurement is made in another electrical mobility range. This procedure is repeated until the density has been determined in the whole range desired.
For the above-presented scanning measurement to produce reliable results, the flow 13a to be analyzed should remain unchanged during the whole measurement operation. Under real measuring conditions, there may be temporal variations in the flow to be analyzed, for which reason the above-presented solution of prior art is poorly suitable for the real-time measurement of a flow containing polydispersive particles under real conditions.